1. Field of the Invention
Electric drive units are used in a multitude of application areas for realizing driving operations of movable components by means of an external force. Various movable components or adjustable elements such as seats, side windows, sun roofs, sliding doors, etc., of motor vehicles are activated by electric drive units.
2. Description of Related Art
Such electric drive units comprise an electric motor, for example a d.c.-motor, for producing and providing electric drive power, a gear for translating the motion of the electric motor, a device for mechanically coupling or adapting the electric motor or the gear to the movable component or adjustable element, particularly for transforming the rotational movement of the electric motor into a linear movement, and an electronic module for controlling and monitoring the electric motor. For example, the r.p.m. and the power of the electric motor are controlled in a closed loop fashion. The driving power required for the electric driving unit lies in general between 100N and 500N.
The driving operations performed by means of an external force or electromotive force are frequently performable automatically in order to increase the comfort for the user or operator. Now the user or operator merely needs to initiate the driving operation and can pursue other activities during the automatic or fully automatic driving operation. However, when driving operations run automatically, particularly automatically running closing operations, that is, when automatic functions are realized for the closing operation, there is the danger of jamming of body parts or objects. This danger is particularly present where the user or operator does not further watch the driving operation or where the user or operator initiates the driving operation from a larger distance by remote control. Due to the substantial risk of injury connected with such jamming, regulations have already been passed which permit, for example automatically running closing operations only under certain preconditions to assure a certain protection against jamming. These regulations determine: the field of application, for example, window lifter and sun roofs of motor vehicles, the maximally permissible jamming force, for example 100 N, the testing bodies to be used for checking the effectiveness of the jamming protection, the testing body characteristics for example the elasticity, the spring rate and the geometry of the testing bodies, and the limiting conditions under which the jamming protection must become effective. In connection with motor vehicles the conditions are that the ignition key is removed and that the closing operation runs automatically.
It is possible to realize a jamming protection by implementing direct methods or indirect methods. In direct methods, the operating distance traversed by the movable component is monitored with regard to potential impediments. Monitoring applies particularly to openings to be closed. Monitoring is performed, for example, by installing switch panels or by optical means. Monitoring entails high costs and is trouble-prone. the jamming force generated when jamming occurs, is monitored in indirect methods. Jamming is assumed to exist if the driving force or a given excess force exceeds a predetermined threshold value or trigger threshold value. Following recognition of a jamming, certain measures are initiated. Particularly, the electric drive unit, namely the electric motor, can be reversed or switched off. The jamming force or excess force evaluated in connection with the indirect methods can be determined directly or indirectly. Direct determination is rather expensive due to the required sensors such as force sensors or torque sensors. Indirect determination requires acquisition and evaluation of measured values of at least one motor parameter that is characteristic for the loading of the electric motor or for the torque moment actually delivered by the electric motor. For example, the following may be evaluated, the drive r.p.m., and/or the current consumption, and/or the power input, and/or the energy consumption of the electric motor. Thus, the jamming force or the excess force is determined by evaluating the effect that the jamming force has on the electric motor of the electric drive unit in terms of a changed load to be driven.
The indirect determination of the jamming force is customarily used for cost reasons. For monitoring a system with an electric motor drive, it is known from German Patent Publication DE 44 42 171 A1 to take the friction force of the drive into account when determining the jamming force during an opening or closing operation. The friction force is ascertained at a point of time during an opening or closing operation when jamming has not yet occurred and a jamming force does not yet exist. The friction force is determined either indirectly or directly. Indirect determination may be performed on the basis of the motor parameters: motor current, or armature current, and/or motor voltage, and/or motor r.p.m. Direct determination is performed through suitable sensors at the electric motor. However, problems arise due to the substantial dependency of the driving force form temperature influences and aging influences and due to the non-reproducible friction characteristic of the electric drive unit. As a result, the jamming force or the excess force is substantially dependent on temperature variations, aging phenomena, and series variations, etc. Thus, for safety reasons and to avoid erroneous evaluations regarding the jamming, the trigger threshold value for the jamming force must be set at least as high as the not reproducible portion of the driving force, and thus of the friction force in the worst case scenario. On the other hand, the electric drive unit is subject to inertia, particularly the electric drive motor is subject to inertia. Therefore, the electric motor keeps running in the original direction for a certain time following recognition of a jamming and the switch-off or reversal of the electrical drive unit as a measure against a further force increase. Thus, for avoiding that a determined maximal jamming force is exceeded, it is necessary to already detect and evaluate as a jamming an excess force that is smaller than this maximal jamming force. For example, it is necessary to detect an excess force in the order of 50 N, so as not to exceed a jamming force of 100 N. In order to assure a jamming protection under these contrary limiting conditions the range of use of the electric drive unit is substantially limited.